(CNN) -- Philadelphia's mayor described his stiff curfew on teenagers' "flash mob" attacks on city residents as a dose of "tough love" and called for parents to keep better watch on their kids.

"I have an understanding of what of our parents are facing, but others are in very challenging situations, and I appreciate that and understand that, but there is no excuse for young people being out so late at night by themselves and then making bad decisions and literally assaulting other citizens," Mayor Michael Nutter told CNN's "American Morning" on Tuesday. "I will not tolerate that."

There have been several assaults by teens on residents in recent weeks, and the beatings have left people badly injured. The city cites the culprits as members of a "flash mob," which is a group of people who decide to gather at a given place via e-mail and social media.

Nutter, who is African-American, delivered a frank sermon at his church, Mount Carmel Baptist in West Philadelphia, on Sunday about the problem.

On Monday, Nutter signed an order reducing curfews to 9 p.m. on Fridays and Saturdays for people under the age of 18 in Center City, the heart of Philadelphia's downtown, and University City, a neighborhood to the west where the University of Pennsylvania and Drexel University are located.

In the rest of the city, the curfew will remain at 10 p.m. for people under age 13 and midnight for people under age 18.

First-time curfew breakers will be fined $150. Repeat offenders could be slapped with fines up to $500, the mayor's office said.

In tandem with the curfew, the city will expand recreation center hours throughout the city.

One example of the violence occurred in late July. Philadelphia police arrested three juveniles and a 19-year-old on assault, reckless endangerment and robbery charges after a mob struck the Center City area.

Two people were injured. The youngest of the juveniles to be arrested is 11, according to Officer Christine O'Brien. The others are 16 and 17; none of them are being identified because of their age.

The 19-year-old was identified as Raymond Gatling, according to the Philadelphia District Attorney's Office.

"We will be prosecuting these young criminals to the fullest extent of the law," District Attorney Seth Williams said in the mayor's Monday news release about the curfew. "There is no excuse for what they did, and they have brought great shame upon themselves and their families."

The problem has overshadowed attention on achievements among youths across the Philadelphia region, one of America's most populous metro areas.

For example, the Philadelphia Youth Poetry Movement won a competition in San Francisco recently, only to return to Philadelphia with little fanfare and news of the flash mobs on the front page.

"It's very frustrating," said Perry "Vision" Divirgilio, a performance coach and poetry movement mentor. "While that's going on, we have kids who are waking up on Saturday to go to a writing workshop. It's hard to keep telling my kids to keep doing well when the ones causing the problems get the attention."

But the issue has emerged as a major topic of conversation.

A Philadelphia Inquirer columnist, Annette John-Hall, called Nutter's church sermon a "good old-fashioned whipping" of the black community. She likes his tough stance on crime, but at the same time, she was disturbed by his remark at the church about race.

"There, he said it. In a way that his white constituents would hear him loud and clear. At that point, he wasn't talking to black people anymore," she wrote.

Nutter said his words at church were well-received. He said "senseless, stupid, ignorant, violent" actions won't be tolerated.

"Teens need to make better decisions. Parents need to step up and take care of their children. We, as a government, cannot raise people's children. You want to have children? You have to take care of them."

Nutter said city authorities are working to be "more anticipatory about things and not just reactive," and are using social media, such as Facebook and Twitter.

But ultimately, Nutter said, the problem stems from home life.

"What are parents doing with their children? Do you know where your child is?"